Saturday, October 17, 2015

real analysis - Is there a way to show the sum of any different square root of prime numbers is irrational?

Is there a way to show the sum of any different square root of prime numbers is irrational? For example, 2+3+5+7+11+13+17+19 should be a irrational number.



One approach I used is to let the sum be a solution of an even polynomial f(x)with integer coefficients and prove by induction that by adding another pk+1. The new polynomial can be written as f(x+pk+1)f(xpk+1)



where f(x+pk+1)=P(x)+Q(x)pk+1,



where P(x) is an even plynomial and Q(x)
is an odd polynomial.




The new polynomial can be written as P2(x)Q2(x)pk+1.



Assume it has a rational solution a, we must haveP(a)=Q(a)=0.



My calculation stopped here since I can't find any contradiction result from this. Can anyone continue this proof, or has other better way to solve this? Thanks!

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